BUILD THE KA1MDA

ICOM CI-V INTERFACE

ICOM's
CI-V protocol is based on a relatively simple bi-directional,
single-wire TTL level signaling system. The single wire consists of a
communications line (DATA) and a ground reference (GND). The DATA line
is held high when there is no traffic. As long as the DATA line is high,
any device can initiate communication by pulling the line low and
sending the Icom attention command (FEFE). When the line goes low, the
other devices see this as a BUSY indication, allowing the single DATA
line to offer some amount of basic flow control. Multiple devices may
connected across the CI-V port in parallel, as long as each device is
set up with a unique hex address. All in all, it's quite elegant in it's
simplicity.

Unfortunately,
Icom charges an exorbitant amount of money for what is basically a
glorified RS-232 to TTL level converter! However, using the MAX-232
integrated circuit, such a converter can be easily built for under $20.
The 2 photos below show the prototype converter I built. I chose to
assemble the unit in a metal enclosure for RFI sheilding. The main
circuit consists of little more than 2 diodes, 5 capacitors, the MAX-232
IC chip, and a voltage regulator chip.

The
converter is normally powered by the host computer's RS-232 port. I
added an extra DC power jack to power the interface externally in case
the RS-232 port's power was insufficient, although this condition has
never occured. I also added 3 LED status indicators for POWER, DATA TX,
and DATA RX to aid in troubleshooting connection problems. Originally,
the DATA TX LED was wired across the TTL side of the converter, while
the RX DATA LED was connected across the RS-232 side of the interface.
The design was later simplified with a single DATA LED across the TTL
side of the interface, which is reflected in the schematic diagram. If
desired, these LED's can be left out all together. As can be seen in the
2 photos above, the interface is quite simple. Component layout and
assembly methods are not critical, as can be seen in my liberal use of
hot-melt glue to hold the status LED's in place on the prototype's front
cover!

Referring
to the schematic diagram below, the interface obtains power from the
host computer's RS-232 port via the RTS and DTR lines on pins 4 and 20.
These lines are always in opposite states- when one is high, the other
is low, and vice-versa. The 2 isolation diodes allow only the positive
voltage from these 2 pins to reach the voltage regulator. The extrnal
power connector also feeds the voltage regulator via its own isolation
diode. The regulator drops the DC voltage down to 5 V for the MAX-232
chip. Serial data is fed to the converter via RS-232 port pins 2 (TX
DATA) and 3 (RX DATA), while pin 7 provides signal ground reference as
well as DC power return. The TTL TX and RX data lines are tied together
at the MAX-232 IC's pins 11 and 12. This common TTL DATA line connects
to the Icom CI-V port.

The
4 capacitors connected to the MAX-232 chip are used by the IC's
internal charge pump, and can be any value between 1uf and 10uf, as long
as the same value is used for all of them. The inductor in series with
the TTL output, along with the .o1uf ceramic disc capacitor across the
TTL output, were added for RFI supression. The value of the inductor is
not critical, I just used one which happened to be laying in my junk
box. I'm not sure if these last 2 components were really needed, but
then again I've never experienced an RFI issue with the converter. As
mentioned previously, the external DC power jack and the status LED's
(along with their associated support circuitry) may be omitted if
desired.

Once
built, testing the unit is easy. Using a terminal program such as
Hyperlink or Procomm, access an unused serial port (baud rate is
unimportant), and make sure local echo is turned off. While typing on
the keyboard, nothing should appear on the screen. Now plug the CI-V
converter into the RS-232 port. If the converter is working, the screen
should now display anything that is typed on the keyboard. This is due
to the TTL RX/TX lines being connected together, which loops all
outgoing RS-232 data back to the host computer through the MAX-232 IC.
With the interface still connected to the serial port and the terminal
program still active, measure the DC voltage at the CI-V connector. It
should read between +3.7 and +5.0 volts referenced to ground. Typing on
the keyboard should cause the voltage level to temporarily drop.

So
what can you do with a CI-V interface? There are a number of logging
and rig control programs which utilize the CI-V interface. One of the
more interesting uses I have discovered involves antenna performance
testing. Using a program called S-METER LITE
by Seed Solutions, it is possible to use the CI-V port to transfer
S-meter readings to the software, which can be used to draw antenna
polar response plots! An example of such a plot is shown above. This
plot shows the polar response of my horizontally polarized KLM 2M-16LBX
yagi using a distant, vertically polarized signal source.